Cellular antennas often include phase shifters for adjusting the phase of signals supplied to or received from radiating elements. Adjustment of phase may be used for electronic steering of beam angle, such as electronic downtilt.
Differential phase shifters adjust the phase between a pair of signal ports. A positive phase shift is applied to one of the ports and a negative phase shift is applied to the other port.
One known type of phase shifter is the “wiper” phase shifter 100 shown schematically in FIG. 1. Signals are received by the phase shifter over an input line 11 and transmitted through the phase shifter to a number of signal ports A1, A2, A3 and A4. Signals are supplied from the ports A1, A2, A3 and A4 to radiating elements A1′, A2′, A3′ and A4′ over feedlines 12.
The input line 11 includes a central annular coupling region 14. This annular conductive region 14 couples capacitively to a conductive wiper 15 which in turn couples capacitively at each end to a conductive arc 16, 17. Thus signals received over the input line 11 are transmitted through the annular coupling region 14 and the wiper 15 to the arcs 16, 17.
The wiper 15 pivots around the point 18 at the centre of the central coupling region 14. Rotation of the wiper around this point alters the path length between the input line 11 and each of the signal ports A1, A2, A3 and A4, thereby introducing phase shifts to signals transmitted to each of those ports.
The arc 16 and the arc 17 are of different radii and are generally both centred on the pivot point 18. These different radii lead to different phase shifts for ports connected to different arcs. For example, in the phase shifter shown in FIG. 1, arc 17 has a smaller radius than arc 16. For the same angle of rotation, θ, of the wiper 15 about the pivot point 18, ports on arc 17 will experience a smaller phase shift than ports on arc 16. Thus, port A1 has a larger negative phase shift than port A2; and port A4 has a larger positive phase shift than port A3.
The Applicant has found that the configuration shown in FIG. 1 introduces undesirable phase errors.
The following is an analysis of the phases of signals supplied to each of the ports, where R1 is the radius of arc 16, R2 is the radius of arc 17, r is the radius of the central annular coupling region 14 and θ is the angle of the wiper 15 relative to a central position.
If we consider port A1, the phase shift includes a component created by a change in the path length in the outer arc. This component is equal to
  -            2      ⁢      π      ⁢                          ⁢              R        1            ⁢      θ        λ  where λ is the wavelength of the signals, and R1θ is of course the length of the outer arc between the central position 20 and the point 21 where the wiper 15 intersects the arc 16.
However, the phase shift also includes a component created by a change in the path length in the central annular coupling region 14. This component is equal to
  -                    2        ⁢        π        ⁢                                  ⁢        r        ⁢                                  ⁢        θ            λ        .  
Applying similar analysis to each port we find that:
            Δφ      ⁡              (                  A          ⁢                                          ⁢          1                )              =                            2          ⁢          πθ                λ            ⁢              (                              -            r                    -                      R            1                          )                        Δφ      ⁡              (                  A          ⁢                                          ⁢          2                )              =                            2          ⁢          πθ                λ            ⁢              (                  r          -                      R            2                          )                        Δφ      ⁡              (                  A          ⁢                                          ⁢          3                )              =                            2          ⁢          πθ                λ            ⁢              (                  r          +                      R            2                          )                        Δφ      ⁡              (                  A          ⁢                                          ⁢          4                )              =                            2          ⁢          πθ                λ            ⁢              (                              -            r                    +                      R            1                          )            
From the above equations it can be seen that the phase shifts provided to the various ports are not symmetric about zero phase shift. That is, Δφ(A1)≠−Δφ(A4) and Δφ(A2)≠−Δφ(A3). Furthermore, the phase shifts introduced between all pairs of adjacent antenna elements cannot be made equal. These are undesirable phase errors which have a negative impact on the performance of an antenna including the phase shifter.
Wiper phase shifters are also generally bulky and therefore unsuitable for some applications.
It is an object of the invention to provide improved antenna performance.
It is a further object of the invention to reduce undesirable phase errors in wiper-style phase shifters.
It is another object of the invention to provide a wiper-type phase shifter with a reduced size.